Feeding regulation is essential to ensure that animals consume calories in proportion to their energy requirements. An imbalance of neuromodulatory systems that regulate feeding may result in obesity and eating disorders, with significant health-related consequences. The long-term objective of this proposal is to increase understanding of the molecular signaling pathways that regulate food intake and how they interact, crucial for devising rational approaches toward controlling obesity and eating disorders. The monoaminergeric neurotransmitters, dopamine and serotonin, oppositely regulate feeding. In mammals, dopamine promotes feeding and serotonin inhibits it. Preliminary studies of this proposal showed that dopamine and serotonin regulate feeding in the fruit fly Drosophila melanogaster and identified the dopaminergic neurons that promote feeding and serotonergic neurons that inhibit feeding. The relative simplicity of the fruit fly nervous system provides a tractable model to dissect how dopamine and serotonin oppositely regulate feeding. Aim 1 will examine whether the activity of serotonergic neurons bidirectionally controls feeding behavior in response to external gustatory cues and internal physiological state. Aim 2 will test the hypothesis that dopamine and serotonin act over short timescales to regulate feeding during a meal as well as over longer timescales to adjust feeding based on internal physiological state. Aim 3 will examine whether dopamine and serotonin are opponent signals that act on the same or different neurons to modulate feeding. The proposed molecular genetic, electrophysiological and behavioral approaches will provide a comprehensive analysis of dopamine and serotonin function that is difficult to achieve in other systems. These studies will provide insight into how the monoaminergic neurotransmitter systems exercise control over feeding and will significantly advance understanding of feeding modulation, an essential foundation relevant for human health and disease.